Prior art workers have devised toggle-type plastic injection molding machines which are hydraulically actuated and which are capable of performing ordinary injection molding cycles and simple coining cycles. In such machines, the hydraulics which activate the plastication and injection assembly of the machine and the toggle-type mold clamping assembly of the machine are inter-related. Such machines are capable of performing conventional injection molding cycles and simple coining cycles since both types of cycles are made up of a plurality of strictly sequential steps. In the ordinary injection molding cycle, the basic steps comprise mold clamping, injection, and mold opening. A coining cycle requires the use of a variable volume mold, as is well known in the art. The simple coining cycle is made up of the following sequential steps: mold close, inject, coin, and mold open.
It has been discovered that if in a coining cycle the coining and injection steps are accurately and repeatably controlled, and if during the coining step the volume of the mold is changed with accurate and repeatable speed and position control, higher quality products are produced. Such higher quality products are characterized by reduced stresses within the plastic material of the product, reduced surface imperfections, the substantial elimination of bubbles and an overall improved strength.
During a coining operation, the linear movement of the movable mold half of the variable volume mold is usually of the order of from about 1/32 inch to about 1/4 inch. To divide such a small distance into a series of positions and to shift the movable mold half from one position to the next at different, controlled speeds requires the use of a toggle-type plastic injection molding machine since the mold position can be determined by determining the position of the cross head of the toggle mechanism, as will be described hereinafter. This is true because the cross head travels with respect to the travel of the movable mold half at a ratio of from about 10:1 to about 20:1.
The usual hydraulically controlled toggle-type plastic injection molding machine of the prior art is not capable of performing the above described precision coining cycle. The present invention provides a hydraulic control system capable of performing ordinary sequential injection molding cycles and simple sequential coining cycles. The hydraulic system is also capable of performing a precision coining cycle since the hydraulic instrumentalities controlling the mold clamping assembly of the machine can be isolated from the hydraulic instrumentalities controlling the injection assembly of the machine so that injection and clamping can be individually and simultaneously controlled with a high degree of repeatable precision.